Hydrocarbon pyrolysis process

ABSTRACT

THE PRESENT INVENTION COMPRISES, IN A PROCESS FOR THE PYROLYSIS OF NAPHTHA, THE IMPROVEMENT CONSISTING ESSENTIALLY OF FEEDING FROM ABOUT 1.5 TO ABOUT 2.2 POUNDS OF STEAM PER POUND OF NAPHTHA FED TO THE PYROLYSIS FURNACE WHILE FEEDING ABOUT 4.0 TO ABOUT 6.5 POUNDS PER MINUTE OF HYDROCARBONS PER SQUARE FOOT OF CROSS SECTIONAL AREA OF SAID FURNACE.

"United States Patent US. Cl. 260-679 R 7 Claims ABSTRACT OF THE DISCLOSURE The present invention comprises, in a process for the pyrolysis of naphtha, the improvement consisting essentially of feeding from about 1.5 to about 2.2 pounds of steam per pound of naphtha fed to the pyrolysis furnace while feeding about 4.0 to about 6.5 pounds per minute of hydrocarbons per square foot of cross sectional area of said furnace.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of our copend ing US. patent application Ser. No. 694,432, filed Dec. 29, 1967 now abandoned.

US. patent application S/N 695,598, filed Ian. 4, 1968 and assigned to the assignee of the present invention, relates to the general field of the pyrolysis of hydrocarbons.

BACKGROUND OF THE INVENTION (1) Field of the invention Processes for thermally cracking hydrocarbons into acetylene, ethylene, and other unsaturated gases in regenerative furnaces have been described at length in the literature. (See, for example, the Wullf process described in US. Pats. 1,996,185; 2,037,056; 2,236,534; 2,236,535; 2,236,555; 2,319,679; 2,518,688; 2,526,696; and 2,908,- 625.) The pyrolysis of methane, ethane, propane, and butane and natural gas to form acetylene and ethylene have particularly been studied. (See, for example, Tropsch and Englofi, Industrial and Engineering, Chemistry, 27, p. 1063 (1935) and US. Pat. No. 1,983,992.) As illustrated by FIG. 1 of US. 2,236,534, such processes typically consist of a regenerative furnace in which the thermal cracking of hydrocarbons is accomplished generally at temperatures above about 1500" F., through carefully controlled contact periods of generally less than about one second. A rapid cooling of the efiluent from the furnace is usually essential to obtaining the desired ethylene and acetylene products. By control of the process conditions, the ratio of acetylene to ethylene can be controlled generally within a substantial range.

(2) Description of the prior art As taught in Acetylene and Its Properties, Manufacture, and Uses, vol. I by S. A. Miller, published by The Academic Press (1965), p. 388 Table V.5 the steamhydrocarbon ratio for the Wulff process and related pyrolysis processes has ranged from about 2.41 for butane up to about 8.03 for propane (all ratios expressed in pounds of steam per pound of hydrocarbon). This same table gives the range for gasoline (77.3 API) at 3.50- 3.65 and jet fuel (55.7 API) at 3.91-4.19. Feed rates utilized in such furnaces have been in the general range of about 3.0 pounds per minute per square foot of cross sectional area of the furnace. Attempts to reduce the pounds of steam fed per pound of hydrocarbons have 3,629,353 Patented Dec. 21, 1971 been found to cause serious loss in yields of the desired C olefins.

SUMMARY General statement of the invention The present invention embodies the discovery that by simultaneously increasing the feed rate as the steam ratio is reduced, the feed rates of conventional Wulif furnaces may be increased by to as compared to the feed rates normally accepted in conventional operations. This increase in feed rate results in a corresponding decrease in the previously accepted capital investment required for Wultf and similar pyrolysis furnaces. Further, this reduction in cost can be accomplished without significant loss in yields of the desired C unsaturated products. Simultaneously with the increase in throughput the quantity of steam required can be reduced by as much as 35%.

Thus, it may be said in summary that the present invention represents improvements in the accepted methods for the pyrolysis of hydrocarbons, preferably hydrocarbon naphthas, e.g. light naphthas, heavy naphthas, and most preferably coker naphthas, by operating at steam ratios of from about 1.5 to about 2.2 pounds of steam per pound of hydrocarbon feed to the furnace and while simultaneously operating at feed rates of from about 4.0 to about 6.5 pounds per minute of hydrocarbons per square foot of cross sectional area of the furnace. Still more preferred ranges are from about 1.6 to about 2.0 pounds of steam per pound of hydrocarbon and feed rates of from about 5.0 to about 6.0 pounds of hydrocarbon per minute per square foot of cross sectional area of the furnace.

Not only are the capital investments thus decreased to levels as low as 57-69% of the previously accepted investment for furnaces per unit of C olefins produced, but in addition the quantity of steam is reduced. This results in savings both in steam generation and in the quench system required for cooling the efliuent gases.

BRIEF DESCRIPTION OF THE DRAWING.

Because of the nature of the invention and the entirely conventional apparatus which is employed therewith, no drawing is provided. Schematic representations of many variations of hydrocarbon pyrolysis processes, including the preferred Wulff process, may be found in the various references cited in the specification of this application. (See, particularly vol. III, Modern Chemical Processes, pp. 260-270, especially FIGS. 4 and 5 for diagrams showing the furnace valve arrangements and feed and exhaust flows for a typical Wulif process and for a flow sheet for the production of acetylene by the Wullf process.)

DESCRIPTION OF THE PREFERRED EMBODIMENTS (1) Starting materials The preferred starting materials for the invention are naphthas, e.g. light naphthas (boiling in the range of from about 70 to about 300 F.); heavy naphthas (boiling in the range of from about 225 to about 450 F.); straight run naphthas (boiling in the range of from about 70 to about 400 F.), and most preferred, coker naphthas (boiling in the range of from about 100 to about 400 F.).

As stated previously, the feed rates for the feed stocks of the present invention, while they may vary somewhat by virtue of differences in molecular weight of the various hydrocarbon feed stocks which can be employed, must in all cases, fall within the range of about 4.0 to about 6.5 pounds of hydrocarbon per minute per square foot of cross sectional area of the furnace in order to achieve the valuable advantages of the present invention without serious sacrifice in the yield of the desired C olefins.

While other feed materials may be employed in conjunction with the above naphthas, it is particularly-preferred that the feed consist of at least 30% by weight of one of the above mentioned naphthas and feeds consisting essentially of one or more of the above naphthas are particularly preferred.

(2) Steam requirements It is essential for the proper operation of the present invention that the steam ratios be in the range of from about 1.5 to about 2.2 pounds of steam per pound of hydrocarbon, and more preferably from about 1.6 to about 2.0 pounds of steam per pound of hydrocarbon.

( 3 Temperature Temperatures for use with the present invention are not narrowly critical and will be those conventionally employed with the Wulff or similar processes. Generally, temperatures in the range of from about 1,000 to about 2,000" C. is the maximum temperature within the furnace.

(4) Pressure Pressure will also be conventional for the Wulff process, and similar pyrolysis processes, but will preferably be in the general range of from about 0.2 to about 0.8 atmosphere, with pressures of from about 0.4 to about 0.5 atmosphere being particularly preferred.

(5) Residence time The residence time will also be the conventionally preferred times for use with the Wulff and similar furnaces, but will generally be in the range of from about 0.2 second or less, with times below about 0.02 second being particularly preferred.

(6) Products produced The products produced will be those generally produced by the Wulif process, with hydrogen, methane, acetylene, and ethylene being major constituents and minor constituents including carbon monoxide, nitrogen, carbon dioxide, benzene, and other aromatics, propyne and allene, vinyland di-acetylene, higher olefins and higher parafiins collectively making up a minor portion of the total product.

(7) Example A conventional Wultf furnace of the general type described in the article beginning at page 260 of Modern Chemical Processes, vol. III and of the general types shown in FIG. 2 of that article, is fed a mixture of steam and coker naphtha. The furnace is 9" by 20" in cross section, providing a nominal cross sectional area of 1.25 square feet. Under conventional operation, when 2.8 pounds of steam per pound of hydrocarbon are fed at a feed rate of 4.5 pounds per minute of hydrocarbon into this furnace, acetylene and ethylene are produced in good yield.

When the quantity of steam is reduced to about 1.8 pounds of steam per pound of hydrocarbon feed, still maintaining the hydrocarbon feed rate at about 4.5 pounds per minute, serious decreases in the yields of C olefins produced are observed.

When 1.8 pounds of steam are fed, together with an increased hydrocarbon feed rate of about 7.0 pounds per minute, yields as good as those obtained in the first de- 4 scribed run at lower rate and higher steam ratio are obtained.

Similarly good yields are obtained at steam ratios of from about 1.5 to about 2.2 so long as feed rates are in the range of from about 4.0 to about 6.5 pounds of hydrocarbon per square foot of cross sectional area of the furnace per minute. Especially good yields are obtained in the range of from about 5 to about 6 pounds per minute of hydrocarbon per square foot of cross sectional area of the furnace where the steam ratio is about 1.6 to about 2.0 pounds of steam per pound of hydrocarbon.

It should be understood that the above is merely illustrative and that the invention is subject to many variations and is to be limited only by the claims appended hereto.

What is claimed is:

1. In a Wulff regenerative process for production of C unsaturated hydrocarbons by the pyrolysis of naphtha in regenerative furnaces, at about 1000 to about 2000 C., 0.2 to 0.8 atmospheres and furnace residence times below about 0.2 seconds in a regenerative Wulff furnace, the improvement consisting essentially of feeding from about 1.5 to about 2.2 pounds of steam per pound of naphtha fed to the pyrolysis furnace while feeding about 4 to about 6.5 pounds per minute of hydrocarbons per square foot of cross sectional area in said furnace.

2. The process of claim 1 wherein the hydrocarbons comprise at least 30% by Weight of a coker naphtha.

3. The process of claim 1 wherein the hydrocarbons comprise at least 30% by Weight of a light naphtha.

4. The process of claim 1 wherein the hydrocarbons comprise at least 30% by weight of a heavy naphtha.

5. The process of claim 2 wherein from 1.6 to about 2.0 pounds of steam are fed per pound of hydrocarbon and wherein the hydrocarbon feed ranges from about 5 to about 6 pounds per minute per square foot of cross sectional area of the furnace.

6. The process of claim 3 wherein from 1.6 to about 2.0 pounds of steam are fed per pound of hydrocarbon and wherein the hydrocarbon feed ranges from about 5 to about 6 pounds per minute per square foot of cross sectional area of the furnace.

7. The process of claim 4 wherein from 1.6 to about 2.0 pounds of steam are fed per pound of hydrocarbon and wherein the hydrocarbon feed ranges from about 5 to about 6 pounds per minute per square foot of cross sectional area of the furnace.

References Cited UNITED STATES PATENTS 2,236,535 4/1941 Hasche 260-679 2,236,555 4/1941 Wulff 26O---679 2,755,321 7/1956 Hasche 260-679 2,645,673 7/1953 Hasche 26O679 2,904,502 9/1959 Shapleigh 208 OTHER REFERENCES Acetylene and Its Properties, Manufacture, and Use, vol. I by S. A. Miller, 1968, pp. 383-388.

DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, JR., Assistant Examiner US. Cl. X.R. 

